1. Field of the Invention
The present invention relates to a method of producing a ferrite material, in particular a Ni--Cu--Zn ferrite material which is suitable for use as a chip inductor.
2. Description of the Related Art
An inductor element has been used as a noise filter in an electric circuit of an electronic device. In order to achieve a compact size and effect a high density attachment, there has been suggested and put into practical use a chip inductor involving only a small amount of a magnetic flux leakage, occupying only a small area, and having a structure in which the inner conductor is disposed within a ferrite ceramic (core).
Such chip inductors, for example, may be obtained by simultaneously calcining a plurality of ferrite material layers and electrically conductive material layers formed between the ferrite layers. In general, there has been used a Ni--Cu--Zn ferrite material as the ferrite material for forming the chip inductors, and, as an electrically conductive material, there has been used a Ag material which has a high electric conductivity.
When Ag is used as an electrically conductive material and the above chip inductor is obtained by means of a simultaneous calcining treatment, the melting point of Ag is 950.degree. C. under the oxygen equilibrium conditions in the atmosphere. If it is heated to a temperature of 900.degree. C. or higher, a plastic deformation of Ag will begin as the time of heating progresses, thereby causing it to penetrate and disperse throughout the ferrite. Because of this, there will occur some problems which can include the cross sectional area of the internal conductor will decrease, the direct current resistance value will increase, and the consumption electric power will thus increase. Moreover, if it is heated to a high temperature which is higher than 950.degree. C., a part of the internal conductor will be disconnected, losing the predetermined function as an inductor. For this reason, it is required that the calcining treatment should be conducted at a temperature of 950.degree. C. or lower, preferably 900.degree. C. or lower, in order to obtain a chip inductor using Ag as an internal conductor.
However, it is necessary to calcine a Ni--Cu--Zn ferrite material which was used as a core material (ferrite ceramic) of a chip inductor at a temperature of 1000.degree. C. or higher so as to obtain a dense calcined body. If the calcining treatment is conducted at a temperature lower than such a value, it is impossible to obtain a sufficient calcining density, hence causing a problem in that the initial magnetic permeability becomes low and pores are adversely created in the calcined body.
Further, in order to effectively remove a noise component having a low frequency of 30 MHz or lower, it is required that, as a characteristic of a noise filter for use in an electric circuit, the cross point frequency at the intersection of an R component frequency curve and the X component frequency curve should be controlled to a value of 10 MHz or lower. For this reason, it is required that the initial magnetic permeability of the Ni--Cu--Zn ferrite material, which is used as a core material of a chip inductor, be maintained at a value of 800 or higher for this purpose.
Furthermore, in order to inhibit a wave form distortion in a frequency component, it is required that, as a characteristic of a noise filter for use in an electric circuit, the cross point frequency at an intersection of an R component frequency curve and an X component frequency curve should be controlled at a value of 5 MHz or lower. For this reason, it is required that the initial magnetic permeability of the Ni--Cu--Zn ferrite material, which is for use as a core material of a chip inductor, be maintained at a value of 1200 or higher.